The Bluefin 21 autonomous sub is hoisted aboard the Australian vessel Ocean Shield on April 1 after testing in the Indian Ocean as search efforts continue for missing Malaysia Airlines Flight 370. / MC1 Peter D. Blair, AP

by Greg Toppo, USATODAY

by Greg Toppo, USATODAY

The unmanned underwater probe searching for evidence of the missing Malaysian airliner is the deepest-rated "autonomous underwater vehicle," or AUV, that the U.S. Navy operates. Less autonomous vehicles in its fleet can go nearly a mile deeper.

Six hours into a planned 16-hour mission Monday, the Bluefin-21 probe aborted its search when a built-in safety feature returned it to the surface, according to the Joint Agency Coordination Center, the Australian government initiative leading the search.

The Bluefin-21, designed to hover about 100 feet above the seafloor, reached its maximum depth Monday, the agency said. The probe is rated to depths of 14,763 feet, or about 2.8 miles.

Other probes, designed to be towed by ships, can reach nearly 3.8 miles underwater, but they're not fully autonomous. Chris Johnson, a U.S. Navy spokesman, said Tuesday that the Navy has several such vehicles but hasn't been asked to send one to the search area off the coast of Australia.

So-called Remotely Operated Vehicles (ROVs) are often tethered to ships with fiber-optic cables that provide video, sonar and other data. They move more slowly than AUVs but can perform special operations underwater.

The chunky, 6,400-pound CURV 21 boasts two arms and "custom tool packages,", according to materials provided by the Navy. Manufactured by Maryland-based Phoenix International, the CURV 21 is one of several ROVs rated to 20,000 feet.

At 1,650 pounds, the Bluefin-21 is much lighter, resembling a slender mini-sub and measuring about 16 feet long. It is to be redeployed once weather conditions improve. The probe didn't snap pictures underwater. It used side-scan sonar to explore the ocean floor for unusual shapes.

Though most side-scan systems can't gauge depth, they can provide searchers with an echo-generated "picture" of the ocean floor, according to the National Oceanic and Atmospheric Administration. If the Bluefin-21 finds anything unusual, it can be outfitted with underwater cameras to photograph what it finds.

The Bluefin-21 is built by Bluefin Robotics of Quincy, Mass. The company was founded in 1997 by engineers from MIT Autonomous Underwater Vehicles Laboratory.

In 2011, a trio of similar vehicles found the wreckage of Air France Flight 447, which had crashed off the coast of Brazil nearly two years earlier. In that crash, three AUVs operated by the Massachusetts-based Woods Hole Oceanographic Institution located the wreckage nearly 2.5 miles beneath the surface of the Atlantic Ocean after about a week of searching. The subs used sonar and underwater photography in the effort, according to Woods Hole.